The turbine section of gas turbine is subjected to high temperature during operation. The rotating portion of the turbine includes a series of wheels arranged on a shaft. The rim of the wheels includes an annular array of dovetail slots configured to receive the dovetail portion of turbine buckets. The turbine buckets extend outward from the rim of the wheel into a hot gas passage through the turbine. The buckets on each wheel form a row of buckets in the turbine. Hot combustion gases flowing through the hot gas passage apply aerodynamic forces to the buckets that cause the buckets, wheels and shaft to rotate to drive a compressor and an external device, such as an electrical generator.
The turbine section includes rows of vanes mounted to a stationary turbine casing. Each row of vanes extends inwardly from the casing into the hot gas passage. Rows of vanes alternate with the rows of buckets. The vanes turn and guide the gases flowing through the hot gas passage into each successive row of buckets.
The hot combustion gases can overheat and damage the turbine bucket. To avoid overheating and heat damage, the buckets are cooled by cooling gases flowing through internal passages in the buckets.
Referring to FIG. 1, a turbine bucket 10 includes an airfoil 12, a shank portion 14, a platform 16, and a root portion 18. The turbine buckets 10 are especially susceptible to deterioration along the platform 16 of the bucket 10.
Turbine bucket deterioration would cause a decrease in the gas turbine efficiency, and would require replacements. Material cost to replace a deteriorated bucket is commonly high, and replacing turbine buckets would require an outage in the gas turbine operation. Hence, the cost to operate a gas turbine would increase due to the replacement material costs.
It has been a continuous search for improvements to resolve durability issues of the turbine buckets. A higher durability of the turbine buckets would require fewer replacements of the buckets, and lower the operation costs of a turbine.
Turbine buckets are generally casted with set number of air duct or cooling passages inside the turbine buckets. Different types of cooling passages are casted into the turbine buckets depending upon cooling needs.
Many have attempted in providing cooling to the bucket platforms. Cooling passages on the platform are generally provided with cooling fluid, such as air flow, that is obtained from an air duct which is casted into the root portion, the airfoil portion, or both the root and the airfoil portions.
Attempts to resolve the issue of turbine bucket platform cooling have been described in, for example, U.S. Pat. Nos. 8,641,368; 8,641,377; 2012/0082567; 7,416,391; 7,309,212; 5,382,135; 2009/0202339; 6,416,284; and J.P. Patent Pub. No. 2008202547.